Direct intramyocardial injection may permit local delivery of protein and gene therapy agents for myocardial and coronary artery disease. However, little is known about the immediate fate of materials administered via percutaneous endomyocardial catheters or viasurgical epicardial injection. Unrecognized loss of injected material can negatively influence the outcome of a preclinical trial. This problem has long been privately discussed by both pharmaceutical developers and developers of drug delivery systems. A recent publication demonstrates that myocardial retention of injected material can vary greatly. We hypothesize that a commercial reagent can be developed to provide both detection and quantitation of cardiac drug delivery systems in in vivo models, as well as provide the means to uniquely identify experimental subgroups dudng double-blind animal trials. In addition, this reagent can be used as a teaching tool and as a means to certify clinicians in myocardial injection techniques. Our Phase I application develop a set of reagents, co-labeled with both a stable isotope and a fluorescent marker. Our Phase II application seeks support to evaluate the usefulness of these reagent in appropriate in vivo models and seeks support to further develop our base technology by the addition of a magnetic resonance imaging (MIR) component.